1. Field of the Invention:
The present invention relates to a method of treating a combustion exhaust gas containing nitrogen oxides. More particularly, it relates to an improved method of removing nitrogen oxides to prevent a pollution by treating a combustion exhaust gas containing nitrogen oxides with ammonia in the presence of oxygen at high temperature wherein a concentration of residual ammonia in the discharged exhaust gas is remarkably reduced.
2. Description of the Prior Arts:
The exhaust gases formed in boilers for combustion of mineral oils such as heavy oils, light oils and crude oils and coals and the exhaust gases formed in internal combustion engines contain nitrogen oxides such as NO and NO.sub.2 (hereinafter referring to as NO.sub.x). NO.sub.x is considered to cause photochemical smog. A concentration of NO.sub.x in the combustion exhaust gas is severely regulated because of preventing the environmental pollution.
Various methods of removing NO.sub.x from combustion exhaust gases have been proposed. As a typical method, the methods of using the solid catalysts have been known. These methods are not always effective in industrial operations because the catalysts are expensive; the apparatus for contacting the gas with the catalyst in high efficiency is required, and the exchange and recovery of the catalyst is periodically required.
In order to overcome these disadvantages, various methods of removing NO.sub.x without using the solid catalyst, have been studied. As the result, the method of selectively reducing NO.sub.x in the presence of ammonia and oxygen, has been proposed (U.S. Pat. No. 3,900,554). In U.S. Pat. No. 3,900,554, the following two methods are briefly disclosed. The first one is a method of selectively reducing NO.sub.x by contacting the combustion exhaust gas containing NO.sub.x with ammonia or an ammonia precursor in the presence of sufficient oxygen at 1600.degree. to 2000.degree. F. (871.degree. to 1093.degree. C.). The second one is a method of selectively reducing NO.sub.x by contacting the combustion exhaust gas with oxygen, ammonia and a reducing agent selected from the group consisting of hydrogen and the other reducing agents at 1300.degree. to 2000.degree. F. (704.degree. to 1093.degree. C.).
These methods are preferable from the viewpoint of the industrial operation because of the broad temperature range in the operation. In order to increase the NO.sub.x removal rate, it is preferable to increase the mole ratio of ammonia to NO.sub.x. However, when the mole ratio of ammonia to NO.sub.x is high, the following disadvantages may be caused. (a) A secondary pollution (toxicity and bad smell) is caused by the unreacted ammonia. (b) Combustion exhaust gases formed by combustion of mineral oils contain sulfur oxides (SO.sub.x) and ammonium salts such as ammonium hydrogensulfate is formed and adhered in a heat-exchanger by the reaction of SO.sub.3 in SO.sub.x with the unreacted ammonia whereby the disadvantages of a corrosion of the heat-exchanger or a clogging of passage are caused.
On the other hand, the inventors have been studied the method of removing NO.sub.x by treating the combustion exchange gas containing NO.sub.x with oxygen and ammonia source at high temperature and the method of removing NO.sub.x under adding hydrogen. As the result, the conditions for imparting high NO.sub.x removal rate are found beside the range of the conditions disclosed in U.S. Pat. No. 3,900,554.
In the second method disclosed in U.S. Pat. No. 3,900,554, hydrogen or the other reducing agent is combined to the combustion exhaust gas. When hydrogen is used, hydrogen causes to disadvantageously decrease the selectively for reducing NO.sub.x with ammonia. When excess of hydrogen is combined, there is a possibility to convert NH.sub.3 into NO.sub.x rather than the desired reaction for reducing NO.sub.x with ammonia. Accordingly, it has been considered that the mole ratio of H.sub.2 /NH.sub.3 is lower than 10 especially lower than 3.
In accordance with the data of Table 3 in the examples of U.S. Pat. No. 3,900,554, the mole ratio of H.sub.2 /NH.sub.3 was 2.4. As the result, the NO.sub.x removal rate was 0.degree. to 1200.degree. F. (648.degree. C.) while NO.sub.x removal was found at 1300.degree. F. (704.degree. C.) and was remarkably found at 1400.degree. F. (760.degree. C.). When the mole ratio of H.sub.2 /NH.sub.3 is constant, the NO.sub.x removal rate increases with increasing amounts of NH.sub.3 charged.